linux-kernel - Re: [PATCH v3 07/15] KVM: MMU: introduce nulls desc

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Message-ID: <20131125142928.GD959@redhat.com>
Date:	Mon, 25 Nov 2013 16:29:28 +0200
From:	Gleb Natapov <gleb@...hat.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
Cc:	Avi Kivity <avi.kivity@...il.com>,
	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>,
	"pbonzini@...hat.com Bonzini" <pbonzini@...hat.com>,
	linux-kernel@...r.kernel.org, kvm <kvm@...r.kernel.org>,
	Eric Dumazet <dada1@...mosbay.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v3 07/15] KVM: MMU: introduce nulls desc

On Mon, Nov 25, 2013 at 12:23:51PM -0200, Marcelo Tosatti wrote:
> On Mon, Nov 25, 2013 at 02:48:37PM +0200, Avi Kivity wrote:
> > On Mon, Nov 25, 2013 at 8:11 AM, Xiao Guangrong
> > <xiaoguangrong@...ux.vnet.ibm.com> wrote:
> > >
> > > On Nov 23, 2013, at 3:14 AM, Marcelo Tosatti <mtosatti@...hat.com> wrote:
> > 
> > <snip complicated stuff about parent_pte>
> > 
> > I'm not really following, but note that parent_pte predates EPT (and
> > the use of rcu in kvm), so all the complexity that is the result of
> > trying to pack as many list entries into a cache line can be dropped.
> > Most setups now would have exactly one list entry, which is handled
> > specially antyway.
> > 
> > Alternatively, the trick of storing multiple entries in one list entry
> > can be moved to generic code, it may be useful to others.
> 
> Yes, can the lockless list walking code be transformed into generic
> single-linked list walking? So the correctness can be verified
> independently, and KVM becomes a simple user of that interface.
> 
The code will become simpler but the problem of never ending walk of
rculist_nulls will remain.

> The simpler version is to maintain lockless walk on depth-1 rmap entries
> (and grab the lock once depth-2 entry is found).
And release it between each rmap walk or at the very end of write
protect?

--
			Gleb.
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